This project will examine the interaction of small peptide inhibitors with the digestive enzyme, pepsin. The structure of these peptides will be based on the zymogen activation peptides produced on conversion of pepsinogen to pepsin under acid conditions. From the 44 residue zymogen tail of porcine pepsinogen, peptides 1-16 and 25-41 have been isolated and characterized with respect to their inhibition of pepsin at pH 5.5. Synthetic analogs of these sequences will be prepared by solid-phase methodology and purified by ion-exchange chromatography. Selective amino acid substitutions will be made to determine the critical structural and conformational features in the interaction. The natural and synthetic peptides will be characterized by the inhibition of pepsin catalyzed milk clotting test. Kinetics and thermodynamics will be studied and compared. The interaction will also be studied by circular dichroism and fluorescence spectroscopy.